JOWETT AND MUFF : GLACIATION OF BRADFORD, ETC. 229 
PHENOMENA OF RETREAT. — THE SOUTH SIDE OF AIREDALE. 
The effects of the shrinkage of the Airedale glacier was 
to open lower and lower cols on each watershed, and thereby 
to lower the levels of the glacier lakes. In this way there 
was formed a number of overflow-channels on each of the spurs 
below the levels of the gaps described above. The shrinkage 
of the ice was not merely intermittent, but oscillatory. On 
most of the spurs there seems to be evidence of three stationary 
periods during the retreat, but the slight re-advances of the 
ice-front, by which deserted channels were again brought into 
operation and cut down lower than before, renders the correlation 
of the dry valleys a matter of great complexity. It will be less 
confusing to describe the valleys taking them spur by spur, 
than to attempt to arrange in sequence the numerous lake- 
levels and outlets during the temporary stationary phases of 
retreat. 
The Outlets of the Bradford Lake. — The first stage in the 
shrinkage of the ice which allowed the Bradford Lake to discharge 
directly at Wibsey Bank Foot has already been described 
(p. 224). A further retreat allowed the cutting of the shallow 
winding channel which runs from Rooley past Bierley Hall into 
the Spen Valley, The level of the Bradford Lake was at this 
period about 635 feet, and probably the upper part of the Leven- 
thorpe delta was now deposited. 
The only other overflow-channel leading out of the Bradford 
basin is at Laisterdyke, a gap through which the Great Northern 
line leaves Bradford. Its intake-level appears to be about 
560 feet. Whilst the Bradford lake remained at this level the 
scarping of the Leventhorpe delta was carried out and the 
Chellow Dean delta deposited (see p. 226). This channel would 
remain in operation till the ice had shrunk to a valley-glacier 
occupying only the main valley of the Aire. 
When the Laisterdyke gap came into operation the over- 
flow of the system of glacier-lakes on the south side of Airedale 
ceased to pass dowTi the Spen Valley. The Laisterdyke gap 
leads into the valley of Farnley Beck, a tributary of the Aire, 
which it joins at Leeds. It is very probable that at one period 
